1. Field of the Invention
The present invention relates generally to cardioversion tachycardia-termination processes, and more particularly, to more nearly optimal cardioversion pulses delivered from a separate capacitor smaller than the defibrillation capacitor where this feature is incorporated in an implantable cardioverter-defibrillator (ICD).
2. Description of the Prior Art
Implantable cardioverter-defibrillator systems now on the market and in use for clinical studies employ capacitors of 120 to 180 microfarads to deliver a defibrillation pulse that usually has an energy of about 20 joules. They use the same capacitor to deliver a cardioversion pulse, and typically, use the same pulse duration for cardioversion as for defibrillation, with the former having an energy in the range of one to five joules. But because the energy employed for cardioversion is some four to twenty times smaller than for defibrillation, the voltage in the former case is correspondingly smaller. Low voltages in the later portions of a pulse are known, however, to be ineffective in terminating tachycardia, and even induce fibrillation. It is worthwhile to provide some background for this observation:
Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical direct current and voltage, in its most primitive form goes back to the last century. [J. L. Prevost and F. Batelli, "Sur Quelques Effets des Descharges Electriques sur le Couer des Mammifers," Comptes Rendus Hebdomadaires des Seances de L'Acadmie des Sciences, Vol. 129, p. 1267, 1899. ] Because of the large currents required for defibrillation, large-area electrodes are employed. [A. C. Guyton and J. Satterfield, "Factors Concerned in Defibrillation of the Heart, Particularly through the Unopened Chest," Am. J. of Physiology, Vol 167, p. 81, 1951.]
For reasons of simplicity and compactness, capacitor-discharge systems are almost universally used in defibrillation. The discharge of a capacitor C through a resistance R results in a curve of voltage versus time (and hence, of current versus time as well) that is a declining exponential function (illustrated by the dotted curve in FIG. 1), with a characteristic time given by the product RC. But it has also been recognized for some time that the long-duration, low-amplitude "tail" of the capacitor-discharge pulse is detrimental. [J. C. Schuder, G. A. Rahmoeller, and H. Stoeckle, "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms," Circ. Res., Vol. 19, p. 689, October 1966; W. A. Tacker, et al., "Optimum Current Duration for Capacitor-discharge Defibrillation of Canine Ventricles," J. Applied Physiology, Vol 27, p. 480, October, 1969.] Although the exact reason for this detrimental effect is not known, plausible speculations exist, with one possibility being that field heterogeneities cause arthythmias in significantly large regions of the heart. [P S Chen, et al., "The Potential Gradient Field Created by Epicardial Defibrillation Electrodes in Dogs," Circulation, Vol. 74, p. 626, September 1986. ] A convenient way to eliminate the low-amplitude "tail" of a capacitor discharge is by switching, which is to say, simply opening the capacitor-load circuit after a predetermined time, or else when voltage has fallen to a particular value, as illustrated by the solid curve in FIG. 1. For this reason, the time-truncated capacitor discharge has been extensively used after its effectiveness was first demonstrated. [J. C. Schuder, et al., "Transthoracic Ventricular Defibrillation in the Dog with Truncated and Untruncated Exponential Stimuli," IEEE Trans. Biom. Eng., Vol. BME-18, p. 410, November 1971.]
Two methods for specifying a time-truncated capacitor-discharge pulse in defibrillation systems have been extensively used. But neither method has involved systematic optimization of the pulse. Some manufacturers such as Medtronic (in their PCD product) simply specify pulse duration (as illustrated by d in FIG. 1), although the physician can choose and adjust the value. A typical value might be a programmable duration of 7 ms. Other manufacturers such as Cardiac Pacemakers (in their Ventak product) specify the relative amount of voltage decline at the time of truncation, with a typical value of the decline being 65% of the initial voltage, as illustrated in FIG. 2. It has become customary to use the term tilt to describe the relative amount of such voltage decline, expressed either as a decimal fraction or a percentage. In algebraic language, EQU tilt=(V.sub.initial -V.sub.final)/V.sub.initial. Eq. 1
As a specific illustration of prior-art practice, one can cite a CPI system that uses a 140-microfarad capacitor for defibrillation. When used with largearea electrodes that typically yield a cardiac electrical resistance of 50 ohms, the system displays an RC time constant of 7 milliseconds. Using the specification of 65% tilt, one obtains a pulse duration of approximately 7 milliseconds, the RC time. But since they (arbitrarily) specify the same tilt specification for cardioversion, and since using the same RC system leads to the same pulse duration, it follows that the final voltage of the cardioversion pulse at the time of truncation can be as low as 40 to 50 volts, depending upon the cardioversion energy chosen. In particular, for a 1-joule pulse from the CPI system, the trailing voltage is 43 volts. Clinical experience with cardioversion pulses having durations in this vicinity shows an effectiveness of only 50 to 80 percent.
In addition to the hazard of supplying such a low voltage to the heart, this prior art constitutes a waste of energy. In tachycardia, heart cells that must be reset are in the state of diastole, a task requiring less energy than resetting cells that are in systole, as in the case of ventricular fibrillation. This fact is reflected in the lower energies typically chosen for cardioversion, but permits a further reduction of the pulse duration without sacrificing effectiveness, and while eliminating the dangerous low-voltage tail from the cardioversion pulse.
A characteristic time associated with far-field diastolic stimulation is in the neighborhood of 1 millisecond. Hence the prior-art cardioversion pulses are dramatically longer than the cardioversion pulses are dramatically longer than the optimum. (Far-field electrodes are relatively large-area electrodes, as distinguished from "point-source" electrodes such as those used in pacing.) The elucidation of this characteristic time employs the concept of chronaxie, that requires some background explanation:
The foundation for defining such a characteristic time is a family of mathematical neurophysiological models for tissue stimulation going back to the turn of the century, with the first important such model having been developed by Weiss. [G. Weiss, "Sur la Possibilite de Rendre Comparable entre Eux les Apparelis Suivant a l'Excitation Electrique," Arch . Ital. de Biol., Vol. 35, p. 413, 1901.] He employed the ballistic-rheotome technique for pulse generation, wherein a rifle shot of known velocity is used to cut two wires in sequence, their spacing being set and measured. Cutting the first wire eliminated a short from a dc source, causing current to flow through the tissue under test, and cutting the second wire opened the circuit, terminating the pulse applied. Converting the electrical data into charge delivered by the pulse, Weiss found that the charge Q needed for stimulation was linearly dependent on pulse duration, d. Specifically, EQU Q=k.sub.1 +k.sub.2 d. Eq. 2.
Subsequently and similarly, the physiologist L. Lapicque collected substantial amounts of data on the amount of current required to for tissue stimulation, using constant-current pulses of various durations. [L Lapicque, "Definition Experimentelle de l'excitabilite," Proc. Soc. de Biol, Vol 77, p. 280, 1909.] Lapicque established an empirical relationship between the current I and the pulse duration d, having the form EQU I=K.sub.1 +(K.sub.2 / d). Eq. 3.
(Note that multiplying this expression through by d yields an expression in charge rather than current, identically the equation given by Weiss. Thus K.sub.1 =k.sub.1 /d and K.sub.2 =k.sub.2 d.) Similar and confirming studies were carried out a few decades later. [H. Fredericq, "Chronaxie: Testing Excitability by means of a Time Factor," Physiol Rev., Vol 8, p. 501, 1928.]
Equation 3 of Lapique shows that the necessary current and the pulse duration are related by a simple hyperbola, shifted away from the origin by the amount of the constant term K.sub.1. Hence the stimulating current required in a pulse of infinite duration is K.sub.1, a current value Lapicque termed the rheobase. Shortening the pulse required progressively more current, and the pulse duration that required a doubling of current for excitation, or 2K.sub.1, he termed the chronaxie, d.sub.c. Substituting 2K.sub.1 and d.sub.c into Eq. 3 in place of I and d, respectively, yields EQU d.sub.c =K.sub.2 /K.sub.1 Eq. 4
Lapicque's model described cell stimulation, rather than defibrillation, but Bourland demonstrated that defibrillation thresholds in dogs and ponies followed the Lapicque model, provided average current is used in the exercise. [J. D. Bourland, W. Tacker, and L. A. Geddes, "Strength-Duration Curves for Trapezoidal Waveforms of Various Tilts for Transchest Defibrillation in Animals," Med. Instr., Vol 12, p. 38, 1978.] In a companion paper, the same workers showed that average current, I.sub.ave, is a useful and consistent measure of defibrillation effectiveness for time-truncated pulses of a given duration through a substantial range of durations, from 2 to 20 milliseconds; in other words, so long as the exponential "tail" is eliminated, pulse effectiveness is not very dependant upon waveform details. [J. D. Bourland, W. Tacker, and L. A. Geddes, "Comparative Efficacy of Damped Sine Waves and Square Wave Current for Transchest Defibrillation in Animals," Med Instr.,Vol. 12, p. 42, 1978.] The defibrillation chronaxie for the heart is usually between 2 milliseconds and 4 milliseconds, as is borne out by a substantial fund of published data. [See co-pending application by Kroll and Smith, Optimal-Pulse Defibrillator.] For cardioversion the chronaxie is approximately 1 millisecond.